Singularly perturbed problems appear widely in many fields, such as fluid mechanics, electronic science, and energy development [1]}, [2]}. The typical characteristic of their solutions is the presence of layers.
To fully resolve layers and obtain uniform convergence with respect to singular perturbation parameters, layer-adapted meshes have been introduced since 1960s [3]} and have been an active research field [4]}, [5]}.
Among them, the Shishkin meshes have been widely used and analyzed [6]}, [7]}, [8]}, because it has a very simple structure.
